The leaves of the trees called aspens flutter in the slightest breeze. Along with the cottonwoods, the aspens are poplars, or trees of the genus Populus. Aspens are native to the Northern Hemisphere, and they grow farther north and higher up the mountains than other poplars. There are three species of aspen. The European aspen (P. tremula) grows in Europe, North Africa, and Asia. The quaking, or trembling, aspen (P. tremuloides) and the big-tooth aspen (P. grandidentata) are native to North America. Aspen trees are valued for their beauty and shade, and they are also used to make paper.